Recent years have witnessed a significant spike in manipulation of liquid droplets because of their applications in microfluidic diagnostics, DNA analysis, drug discovery, microreactors and biosensing. Particularly, droplet manipulation on super-repellent surfaces (i.e., surfaces that are extremely repellent to liquids) has been widely studied because droplets exhibit high mobility, minimal contamination and minimal sample loss on super-repellent surfaces. Various droplet manipulation methods including electric fields, magnetic fields, guiding tracks, and wettability gradients, have been developed to enable the transportation, trapping, merging and splitting of droplets on super-repellent surfaces. However, there are very few studies that demonstrate droplet sorting (i.e., systematically ordering or categorizing droplets by a physical property of the droplet) on super-repellent surfaces.
Accordingly, there is a need for inexpensive and energy-efficient analytical devices for personalized point-of-care diagnostic platforms, lab-on-a-chip systems, biochemical assays and biosensors. This problem is solved by utilizing tunable superomniphobic surfaces with flower-like TiO2 nanostructures to fabricate a simple device with precisely tailored surface energy domains that, for the first time, can sort droplets by surface tension. We envision that our methodology for droplet sorting will enable inexpensive and energy-efficient analytical devices that can conveniently perform diagnostic tests and provide quick results.